PR to enhance the `Usage` object and related logic, to support more
granular token accounting, matching the details available in the [OpenAI
Responses API](https://platform.openai.com/docs/api-reference/responses)
. Specifically, it:

- Adds `input_tokens_details` and `output_tokens_details` fields to the
`Usage` dataclass, storing detailed token breakdowns (e.g.,
`cached_tokens`, `reasoning_tokens`).
- Flows this change through
- Updates and extends tests to match
- Adds a test for the Usage.add method

### Motivation
- Aligns the SDK’s usage with the latest OpenAI responses API Usage
object
- Supports downstream use cases that require fine-grained token usage
data (e.g., billing, analytics, optimization) requested by startups

---------

Co-authored-by: Wulfie Bain <[email protected]>

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[//]: # (BEGIN SAPLING FOOTER)
* openai#732
* openai#731
* __->__ openai#730

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[//]: # (BEGIN SAPLING FOOTER)
* openai#732
* __->__ openai#731

…penai#732)

Fix for openai#734